1.Extract the zip to your guest OS X desktop (not into a folder, literally to your desktop) this means there should be a file on your desktop called AppleAC97Audio.kext?2. Open Terminal, (/applications/utilities/terminal.app)?3. Type sudo -s in terminal then your admin password following this, now you should be root.?4. Type sudo mv /users/(YOUR USERNAME)/desktop/AppleAC97Audio.kext /system/library/extensions ?at this point the file AppleAC97Audio.kext should have disappeared from the desktop,?5. Type sudo chmod -R 755 /System/Library/Extensions/AppleAC97Audio.kext?6. Type sudo chown -R root:wheel /System/Library/Extensions/AppleAC97Audio.kext?7. Now, CLOSE TERMINAL ?then go to /system/library/caches/com.apple.kext.caches/ and trash? Extensions.mkext(This cache will regenerate automatically on boot with the current kext library)8. Reboot and enjoy sound support!

Note: This is just the source and requires XCode to compile, theres some issues with the audio quality that came with the 64-bit port that I haven't the time to track down so i'm putting it up to the community and the VirtualBox devs to take a crack at it, the important thing is this source is full 32/62-bit compliant.